Understanding Linux Memory Cache: Buffers, Cache, Metrics, and Monitoring Tools

1. In‑memory cache

Linux memory cache consists of buffers and cache, which can be observed with the free command.

# 注意不同版本的free输出可能会有所不同
$ free
              total        used        free      shared  buff/cache   available
Mem:       8169348      263524    6875352         668   1030472    7611064
Swap:            0          0          0

Buffer caching is performed directly on raw disk block devices by programs, while cache data passes through the filesystem (ext, xfs, etc.) before being cached.

buffer: program uses raw disk block device, caching data.

cache: data goes through the filesystem and is then cached.

2. Cache metrics

Cache utilization is measured by the hit‑rate metric; a higher hit rate indicates more effective cache use and better application performance. Buffers and cache dramatically improve disk I/O by bridging slow disk access with fast memory. To drop caches, run:

# 清理缓存
$ echo 3 > /proc/sys/vm/drop_caches

3. Tools for analyzing cache

cachestat monitors Linux page‑cache hit rates. The command cachestat 1 3 samples every second for three intervals. Example output and field meanings are:

root@test:~# cachestat 1 3
    HITS   MISSES  DIRTIES HITRATIO   BUFFERS_MB  CACHED_MB
   14076        0        0 100.00%        1084     484807
       0        0        0   0.00%        1084     484807
       0        0        0   0.00%        1084     484807

HITS – number of cache hits.

MISSES – number of cache misses.

DIRTIES – number of dirty pages added to cache.

HITRATIO – cache hit rate.

BUFFERS_MB – size of Buffers in megabytes.

CACHED_MB – size of Cache in megabytes.

cachetop displays per‑process cache statistics, including hits, misses, dirties, and read/write hit percentages.

17:56:37 Buffers MB: 1084 / Cached MB: 484808 / Sort: HITS / Order: ascending
PID   UID   CMD               HITS MISSES DIRTIES READ_HIT% WRITE_HIT%
12210 root pg_watcher_serv   15   0      0      100.0%    0.0%
14835 root cachetop          31   0      0      100.0%    0.0%
14880 root ping              359  0      0      100.0%    0.0%
14879 root ping              376  0      0      100.0%    0.0%
14878 root ping              418  0      0      100.0%    0.0%
14881 root nc                481  0      0      100.0%    0.0%

Both tools require the bcc-tools package:

sudo apt install -y bcc-tools